In the part, to produce a thermoelectric conversion material (Bi2 Te3 alloy etc.), the practice had been to add a reducing agent (BaBH4 etc.) into a solution of a compound of the component elements (BiCl3, TeCl4, etc.) to cause the component elements to precipitate as composite nanoparticles (particle size: tens of nanometers or so or less), to alloy the composite nanoparticles by hydrothermal synthesis, and to sinter the alloy powder to obtain a thermoelectric conversion material (Bi2Te3 alloy etc.)
However, this method had the problems of the following points 1), 2), and 3):
1) Impurities (Na, B, etc.) which are derived from the reducing agent (BaBH4 etc.) remain resulting in deterioration of the thermoelectric conversion characteristic of the final product. To remove the impurities, it is necessary to wash the composite nanoparticles before the hydrothermal synthesis, but complete removal is difficult. The level of impurities after washing is not constant, so in the end fluctuations in the thermoelectric conversion characteristic are unavoidable.
2) In the sintering process, among the component elements (Bi, Te, etc.), easily vaporizing elements (Te etc.) are lost due to vaporization, so the targeted alloy composition (Bi2Te3 etc.) cannot be realized and the inherent thermoelectric conversion properties cannot be obtained. As a countermeasure against this, it may be considered to add a larger amount of the easily vaporizing element anticipating the amount of vaporization loss. However, for example, Te is expensive, so a drop in yield should be avoided in terms of cost. Inherently, the extent of loss due to vaporization is not constant, so in the final analysis it is difficult to stably obtain the target composition.
3) Furthermore, there is the defect that the hydrothermal treatment which is required for alloying the composite particles invites an increase in energy and complication of the production process.
On the other hand, it is known to use the solution plasma method to prepare nanoparticles. PLT 1 discloses the method of causing the generation of plasma in an aqueous solution of a metal salt so as to form particle size 500 nm or less metal nanoparticles. The formed nanoparticles are gold, silver, rhodium, and platinum. Further, PLT 2 discloses a method of spraying microparticles which are formed using the solution plasma method together with a jet of multi-arc plasma so as to coat objects (not nanoparticles).
Furthermore, PLT 3 discloses a thermoelectric conversion material which is comprised of a core/shell structure which has a plurality of core parts and a shell part which covers these core parts. The examples show a core/shell type thermoelectric conversion material comprised of core parts of nanoparticles (3 nm) of ZnO oxides which are covered by a shell part of a CoSb3 thermoelectric conversion material. The cores function as phonon scattering particles which raise the thermoelectric conversion characteristic. The solution plasma method is not alluded to.
None of these can solve the above problems 1) to 3).